title: Hijacking of leguminous nodulation signaling by the rhizobial type III secretion system.
abstract: Root-nodule symbiosis between leguminous plants and nitrogen-fixing bacteria (rhizobia) involves molecular communication between the two partners. Key components for the establishment of symbiosis are rhizobium-derived lipochitooligosaccharides (Nod factors; NFs) and their leguminous receptors (NFRs) that initiate nodule development and bacterial entry. Here we demonstrate that the soybean microsymbiont Bradyrhizobium elkanii uses the type III secretion system (T3SS), which is known for its delivery of virulence factors by pathogenic bacteria, to promote symbiosis. Intriguingly, wild-type B. elkanii, but not the T3SS-deficient mutant, was able to form nitrogen-fixing nodules on soybean nfr mutant En1282. Furthermore, even the NF-deficient B. elkanii mutant induced nodules unless T3SS genes were mutated. Transcriptional analysis revealed that expression of the soybean nodulation-specific genes ENOD40 and NIN was increased in the roots of En1282 inoculated with B. elkanii but not with its T3SS mutant, suggesting that T3SS activates host nodulation signaling by bypassing NF recognition. Root-hair curling and infection threads were not observed in the roots of En1282 inoculated with B. elkanii, indicating that T3SS is involved in crack entry or intercellular infection. These findings suggest that B. elkanii has adopted a pathogenic system for activating host symbiosis signaling to promote its infection.
authors.affiliation: Department of International Environmental and Agricultural Science, Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan.
authors.forename: Shin
authors.initials: S
authors.lastname: Okazaki
authors.forename: Takakazu
authors.initials: T
authors.lastname: Kaneko
authors.forename: Shusei
authors.initials: S
authors.lastname: Sato
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Saeki
mesh_terms: D058947:Bacterial Secretion Systems / Q000502:physiology*; D020369:Bradyrhizobium / Q000502:physiology*; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology; D018506:Gene Expression Regulation, Plant / Q000502:physiology; D009154:Mutation; D009586:Nitrogen Fixation / Q000502:physiology; D027343:Rhizome / Q000235:genetics / Q000378:metabolism / Q000382:microbiology; D053204:Root Nodules, Plant / Q000235:genetics / Q000378:metabolism / Q000382:microbiology; D013025:Glycine max / Q000235:genetics / Q000378:metabolism / Q000382:microbiology; D013559:Symbiosis / Q000502:physiology*
chemical_list: D058947:Bacterial Secretion Systems
keywords: effector; pathogenesis; plant disease resistance; symbiotic nitrogen fixation